FIG. 1 shows a starter motor D which includes an electric motor M connected between ground and a power supply terminal B+ at battery voltage.
A contactor 1 connected between said terminal B+ and the electric motor M controls the supply of power to the motor.
The contactor 1 is a relay with a moving core (not shown) actuated by an actuator coil 2 and a latching coil 3 respectively for pushing a power contact la into a closed position and for holding it there.
The actuator coil 2 is connected between the coil 3 and the side of the motor M that is not connected to ground. The opposite end of said coil 3 is connected to ground.
At their common end, the coils 2 and 3 are connected to the source of a transistor 4, for example a field effect transistor the drain of which is connected to the terminal B+ via the starter switch 6.
A microprocessor 5 also connected to the power supply terminal B+ applies a control voltage to the gate of the transistor 4 to control the transistor 4 on an on/off or progressive basis. As shown in FIG. 1, for example, the microprocessor 5 is integrated with the relay 1 and the transistor 4 in the starter motor casing. It can equally well be external of the starter motor, anywhere on the vehicle.
A starter motor of the above kind evidently has the advantage of being self-contained, that is to say of requiring no electrical connections other than those used by standard, non-electronic starter motors, namely a control cable for connecting it to the starter switch 6, a cable for supplying power to the motor M and a ground return via the starter motor casing.
The microprocessor 5 controls a number of functions. In particular, it stops the starter motor automatically when the vehicle engine starts. It also provides various protection functions; for example, it cuts off the supply of power to the starter motor if the motor M is overloaded or it runs too fast; it also cuts off the supply of power to the starter motor in the event of supply voltage surges.
The protection functions prevent some tests being carried out on the starter motor.
For example, to measure the speed characteristic of the starter motor for a given opposing torque, the starter motor must be supplied with power under stable conditions for several seconds, which leads to voltage or current variations. If the microprocessor is programmed to detect malfunctions by monitoring voltage or current variations, it stops the starter motor in a few tenths of a second. The measurement is then impossible.
A first solution which might be envisaged would be to provide an ancillary circuit for energizing the coil or coils of the contactor in parallel with the transistor 4 during measurements.
However, this solution would necessitate an additional terminal on the starter motor and additional wiring. This would result in a significant increase in cost and overall size.